Sound strategy for levitating and moving particles
DOI: 10.1063/PT.3.2712
Bounce a propagating sound wave off an acoustic mirror and the superposition of original and reflected waves may yield a standing pressure wave. A particle located in that pressure field will feel a force driving it to a position of stable equilibrium; indeed, an acoustic standing wave with sufficient amplitude can even suspend a particle against the action of gravity. Numerous researchers have used acoustic levitators to study behaviors of liquid drops without the complications of a confining vessel. The problem with conventional levitators, though, is that the separation between the wave generator and the reflector must be resonantly tuned if a standing wave is to result. Now Marco Andrade of the Institute of Physics at the University of São Paulo, Brazil, and colleagues have demonstrated a device that can levitate particles— indeed, manipulate them—without the need for fine tuning. Their trap, based on a decades-old proposal by Charles Rey, is shown here suspending four polystyrene balls. It comprises a 10-mm-diameter cylindrical transducer (at the tops of the photos) that generates the sound, and a somewhat larger concave reflector. For the coaxial geometry shown on the left, the trapped balls remain at a fixed distance from the mirror even as the mirror–transducer separation varies from 50 mm to 100 mm. Moreover, as one misaligns the axes of the transducer and mirror (right
